Field emitter device having porous dielectric anodic oxide layer

ABSTRACT

A field emitter device comprises a dielectric anodic aluminum oxide layer having pores with wires the front ends of which constitute individual field emitting cathodes, a gate eleectrode overlying a front surface of the layer, and an address electrode overlying a back surface of the layer and in electrical contact with the wires. The problem of short circuit between the gate electrode and the field emitter is overcome by cleaning the pore walls adjacent the gate electrode and/or by selectively dissolving the back ends of individual wires.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to a field emitter device for use in flatpanel displays, etc.

2. Description of Related Art

Electron field emitter structures have potential application in manydifferent areas, for example to produce flat panel displays (FieldEmitter Displays or FED's) and high frequency electronic devices. Ineach case the device would consist of an array of many individual fieldemitting cathodes. In most cases it is desirable that

i) electron emission occurs at low voltages,

ii) high emission currents per unit area of the device can be achieved,and

iii) the capacitance between the cathode and the gate electrode is keptto a minimum.

iv) leakage between cathodes and gate layer is minimised.

The practical reasons for i) and ii) is that this determines thetransconductance of the device and hence its maximum operating speed--animportant parameter for high speed devices. In addition for FED's,operation at low voltages is attractive since this will lower the costof the driving electronics used to control image generation. Whilst ii)is advantageous because if many emitters are fabricated per unit areathis will tend to average the "flicker noise" that may be associatedwith each individual emitter so leading to a "cleaner" image.

The reason for iii) is that this also determines the maximum speed atwhich a device may work and also for relatively low frequencyapplications such as FED's driving the capacitance of the dielectriccontributes substantially to the overall power consumption of the unit.

The reason for iv) is that leakage between the cathodes and gate layerincreases the power consumption of the device and also makes uniformdisplay brightness for a FED harder to achieve. This is because currentflowing between cathode and gate will not contribute to exciting thedisplay phosphors and thus those areas of a display having large leakagecurrent will be less bright for a given total cathode current.

In order that factors i) to iv) may be realised an ideal field emittergeometry may be postulated. This would comprise i) a very small cathodeto gate spacing so as to minimise the applied voltage required toproduce field emission. ii) A very high density of field emitters perunit area each capable of delivering substantial current. iii) A thickdielectric separating the gate electrode from the substrate. iv) Noelectrical contact between the emitters and gate layer.

When aluminium is anodised in an electrolyte having some dissolvingpower for the oxide, there results a porous anodic aluminium oxide filmwhich may be regarded as consisting of an array of hexagonal cells witha pore in the centre of each cell. When removed from the aluminium metalsubstrate on which it was formed, this anodic oxide film forms apotentially excellent dielectric layer for such field emitterstructures. U.S. Pat. No. 5,164,632 describes an electron emittingelement for use in a display device, in which the pores of an anodicaluminium oxide film are filled with metal and constitute electronemitting members, and a gate electrode disposed on one surface of theinsulator and having protrusions which protrude into each of the pores.Although in principle the electron emitting members are not inelectrical contact with the gate electrode, in practice short circuitsare probable and no means is disclosed for dealing with the problem.U.S. Pat. No. 5,315,206 describes similar and related devices.

This invention is based on the realisation that short circuits betweenthe gate electrode and individual field emitter electrodes is a majorproblem when using dielectric layers of anodic aluminium oxide, and onewhich must be addressed in order to produce a usable device. There areseveral related problems:

i) Each individual field emitting cathode that is in electrical contactwith the gate electrode does not emit any signal. Since the number ofpores in an anodic aluminium oxide film is large, at least about 10⁸pores cm⁻² it is possible to tolerate a situation in which a substantialnumber of individual field emitters is non functional and still get anacceptable signal. Nevertheless, if the problem of short circuits is notaddressed, it can be found that more than 99%, e.g. more than 999 perthousand, individual field emitting cathodes are non-functional.

ii) To obtain field emission from these devices, a substantial voltageis applied between the gate electrode on one surface of the anodic oxidelayer and a conducting substrate constituting an address electrodeoverlying the other surface. These potentials may be of the order ofvolts or tens of volts. When there is a short circuit, through anindividual emitter cathode, between the gate electrode and the addresselectrode, the current that passes may be sufficiently heavy to causelocal damage to the device.

iii) When there is a short circuit, through an individual field emittingcathode, between the gate electrode and address electrode, the leakagecurrent between cathode and gate electrode is increased.

iv) The leakage current between the gate electrode and the addresselectrode through the short circuit may be sufficiently large so as toreduce locally the voltage difference between gate and cathodes due tovoltage drops caused by the large current flowing through the gatelayer, address layer and the rest of the structure. This voltage dropmay be sufficient to prevent other cathodes in the surrounding areaemitting.

It is an object of this invention to provide field emitter structures inwhich these problems are minimised or overcome.

SUMMARY OF THE INVENTION

In one aspect the invention provides a field emitter device comprising adielectric anodic metal oxide layer having a front surface and a backsurface, an array of pores extending through the anodic metal oxidelayer from the front surface to the back surface, the pores containingwires having back ends and front ends constituting individual fieldemitting cathodes, a gate electrode overlying the front surface of theanodic metal oxide layer, and an address electrode overlying the backsurface of the anodic metal oxide layer and in electrical contact withthe back ends of the wires, wherein there is a low or zero incidence ofshort circuits between the address electrode and the gate electrode.

If there is a high incidence of short circuits between the addresselectrode and the gate electrode, the device will not work, for variousreasons given above. The maximum permissible proportion of shortcircuits depends upon various parameters of the device, but it can besafely said that, if there are short circuits through as many as 10% ofthe wires in the pores of the dielectric layer, the device will notfunction. Usually the proportion of wires providing short circuits needsto be kept below 1%. The inventors have measured resistance between gateelectrodes and address electrodes of devices made by them and get valuesin the range 2 to 20 Mohm mm⁻², which suggests that the proportion ofshort circuits is certainly less than 0.1%, and most probably in therange of 1 in 10⁵ to 10⁶, of the wires in the dielectric layer.

Preferably a resistive layer is present between the back ends of thewires and the address electrode. The provision of a resistive layer in asimilar context is described in U.S. Pat. No. 4,940,916. This layerserves two purposes. First it limits the current that can flow throughany shorted field emitting cathode so that the total leakage currentbetween the gate electrode and the address electrode is kept small.Second, the resistive layer serves to ballast the individual fieldemitting cathodes, so that as they begin to emit current as thepotential applied to the gate electrode is increased, an increasinglylarge proportion of the applied voltage is dropped across the ballastresistor rather than the emitter themselves. In this way the currentemitted by each individual field emitting cathode is limited so that thegood emitters which begin emitting at low applied voltages are notdestroyed due to excess emission before slightly poorer emitters emit athigher voltages. Thus a larger proportion of emitters emit and a totalcurrent produced by the array is larger.

It will be understood that, notwithstanding the presence of a resistivelayer, the back ends of the wires in the dielectric layer are deemed tobe in electrical contact with the address electrode. It is envisagedthat the resistance is preferably in the range of 10-100 Mohm per fieldemitting cathode. Depending on the diameters of individual wires, and onthe thickness of the resistive layer, this in turn implies that theresistive layer may need to have a resistivity in the range 10-10⁴ ohmcm. The thickness of the resistive layer is envisaged as of the order of1 μm, e.g. 0.1 μm up to 10 μm or more. Examples of materials for theresistive layer are indium oxide, tin oxide, ferric oxide or zinc oxide,alone in doped form or in admixture. Preferred materials are silicateddiamond-like-carbon produced by ion-beam PVD deposition to form a densewell-adhered amorphous carbon coating having a resistivity of about 50ohm cm to about 2000 ohm cm, MEH-PPV(p-poly(2-methoxy-5-(2-ethylhexoxy)-phenylenevinylene)) and doped orundoped amorphous or polycrystalline silicon.

The gate electrode overlies the front surface of the anodic metal oxidelayer. When the gate electrode is formed, it is quite difficult toensure that the metal being applied does not enter the pores. But gateelectrode metal within the pores and overlying the walls thereof, is amajor source of short circuits. It is therefore a preferred feature ofthis invention that metal of the gate electrode is substantially notpresent within the pores. More particularly, the wall of an individualpore intermediate the gate electrode (overlying the end of the pore) andan individual field emitting cathode (within the pore) is preferablyfree of any conducting material. Techniques for achieving this aredescribed below.

Despite all precautions, inevitably on occasion an individual fieldemitting cathode will be in electrical contact with the gate electrode.Preferably the back end of the wire associated with such individualfield emitting cathode is not also in electrical contact with theaddress electrode. A technique for achieving this is described below

Preferably an individual field emitting cathode is pointed and is spacedfrom the walls of the pore and from the gate electrode. The pointedfront end of a cathode constitutes an emitter cone, and can readily beprovided by means of the Spindt or similar process [See for example (andlater works by same author) C. A. Spindt et al., J. Appl. Phys. 47, 5248(1976)]. Preferably metal of the emitter cone is substantially notpresent overlying the walls of the pore.

In another aspect the invention provides a method of making a fieldemitter device by the steps of

a) providing a dielectric anodic metal oxide layer having a frontsurface and a back surface and an array of pores extending through theanodic metal oxide layer from the front surface to the back surface,

b) providing wires in the pores having back ends and front ends toconstitute individual field emitting cathodes,

c) providing a gate electrode overlying the front surface of the anodicmetal oxide layer, and

d) providing an address electrode overlying the back surface of theanodic metal oxide layer and in electrical contact with the back ends ofthe wires, characterised by taking one or both of the following steps toreduce the extent of short circuits between the address electrode andthe gate electrode:

i) subjecting an intermediate product of step c) to a liquid whichcleans pore walls intermediate the individual field emitting cathodesand the gate electrode,

ii) subjecting an intermediate product of step c) to electrolytic actionto selectively dissolve the back end of any wire that is in electricalcontact with the gate electrode.

BRIEF DESCRIPTION OF THE FIGURES

Reference is directed to the accompanying drawings, in which:

FIG. 1 is a diagrammatic section, not drawn to scale, through apreferred field emitter device according to the invention.

Each of FIGS. 2 to 13 is a corresponding drawing showing a stage in theproduction of the device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows a field emitter device comprising a dielectric anodic metaloxide layer 10 having a front surface 12 and a back surface 14, and anarray of pores 16 extending through the layer from the front surface tothe back surface. Each pore contains a wire 18 having a back to end 22and a front end constituting an individual field emitting cathodecomprising an emitter cone 20. A gate electrode 24 overlies the frontsurface of the anodic metal oxide layer. A resistive layer 26 overliesthe back surface of the anodic metal oxide layer. An address electrode28 overlies the resistive surface and is in electrical contact (throughthe resistive layer) with the back ends of the wires. The emitter cone20 of two of the three individual field emitting cathodes shown isspaced from the gate electrodes and from the pore walls which areconcave at 39. In the left hand pore, a particle 32 has brought thefield emitting cathode into electrical contact with the gate electrode.In this pore, the back end of the wire 18 has been selectively dissolvedaway at 34 and is not in electrical contact with the address electrode28. Thus in the embodiment shown, there are no short circuits betweenthe address electrode 28 and the gate electrode 24.

In the preferred embodiment described below, the dielectric layer 10 isan anodic aluminium oxide layer; wires 18 are of nickel having emittercones 20 of molybdenum, and the gate electrode 24 is of niobium. Theresistive layer 26 may be for example of amorphous silicon. The addresselectrode 28 may be of any of a variety of metals e.g. Al, Ag, Cr, W,Nb, Ta or Ti.

The method of the invention starts with a free-standing dielectricanodic metal oxide layer. Free-standing anodic aluminium oxide films areavailable for purchase in a form convenient for use, for example fromWhatman plc under the trademark ANOPORE. Film thickness is not ofcritical importance, provided that the film is sufficiently robust andmay conveniently be in the range 10-150 μm. Pore diameter and porespacing are not very critical, but pore spacing needs to be sufficientto permit the pores to be cleaned (see steps 6 and 10 below) withoutcausing the whole structure to collapse. The inventor had found itconvenient to use an asymmetric Anopore membrane having a thickness of60 μm, a pore diameter of 0.16 μm adjacent the back surface and a porediameter of 0.02 μm adjacent the front surface (the small diameter poresare removed during processing as described below). Alternatively, it ispossible to make anodic aluminium oxide films and to give them specialproperties which may be of advantage in these field emitter devices.

When aluminium is anodised in an electrolyte having some dissolvingpower for the oxide, there results a porous anodic aluminium oxide filmwhich may be regarded as consisting of an array of hexagonal cells witha pore in the centre of each cell. The diameter and spacing of the poresdepends on the anodising voltage; when this is X Volts, the porediameter is typically about X nm and the pore spacing about 2.5X nm.Between the bottom of the pores and the metal/oxide interface is abarrier layer of thickness about X nm. The total thickness of the porousanodic oxide layer increases with increasing anodising time Thusanodising conditions, including time, voltage and electrolytecomposition and temperature, can be chosen in known manner to create ananodic oxide film of chosen thickness containing a uniform array ofpores of chosen diameter and spacing.

For present purposes, the thickness of a free standing porous anodicoxide film should preferably be greater than the pore diameter, often bya factor of 10 or 20 or even substantially more.

In one preferred embodiment, the device of this invention is formed byapplying a thin film of aluminium (or other anodisable) metal to aconducting substrate, and subjecting the film to anodising conditionsuntil the whole has been converted to an anodic oxide structure at least0.5 μm thick with pores extending all the way through. The desired fieldemitter device can be built round this structure without removing theanodic oxide film from the conducting substrate which can serve as anaddress electrode. Alternatively, it is possible to make an anodicaluminium oxide film containing a barrier layer, and then to thin thebarrier layer, but stopping the process while the anodic oxide film isstill held on its substrate, thus producing a structure in which thepores of the anodic oxide layer are in electrical contact with theconducting substrate.

For the method described below, it is necessary to remove, the barrierlayer, and there are various ways of doing this. One technique isdescribed in Alcan EP 178 831B. This is a voltage reduction techniquewhich is described as thinning and eventually removing the barrier layerso that the anodic oxide film floats free of the metal substrate onwhich it was formed.

Another anodising technique is available to produce improved results. Inan array comprising a large number of individual field emittingcathodes, it is almost inevitable that one or more of such cathodes willshort circuit with its associated gate metal layer. During formation ofthe porous anodic oxide layer described above, it is possible togradually to reduce the anodising voltage for a limited period duringthe anodising operation and then gradually increase it again. Since porediameter is proportional to anodising voltage, the gradual voltagereduction results in pore branching with the resultant pores each havinga reduced diameter. The gradual voltage reduction may be performed insteps as described in Alcan EP 178 831 B although in the present casethe voltage reduction process is preferably stopped well before the filmhas separated from the metal substrate. For example the reductionprocess might be stopped at anodising voltages as high as 50 V whichwould yield ≈50 nm diameter pores. Continued anodisation at reducedvoltage leads to a region of the anodic film with narrower porespropagating towards the metal surface. As the anodisation voltage issubsequently gradually increased, which may also be done in stages, thediameter of the propagating pores increases and some pores terminate. Ifthe anodising voltage is increased to a value similar to that usedbefore the voltage reduction process commenced, the resulting anodicoxide film will consist of four regions. In the region I closest to thefree surface of the film uniform, straight, parallel pores propagateinto the depth of the film. In the next region II, produced duringvoltage reduction, the pores branch and constrict. If a period ofconstant reduced voltage is used then this region will contain a regionof straight, uniform pores of reduced diameter. In the next region III,produced as the voltage is increased, the pores widen and someterminate. Whilst in the last region IV the pores have a similardiameter and density to region 1. The key feature of this structure isthat each pore in region IV is connected to one pore in region I bymeans of only one, or very few, narrow pores in regions II and III. Whenthese pores are filled with metal (see Step 1 below), the constrictioncaused by the narrow pores linking pores in region I to pores in regionIV may cause the resulting metal "wires" to fuse in the event of a shortcircuit with the gate metal layer and thus to minimise damage resultingfrom the short circuit. Each constriction thus acts as a nano-scale fusecapable of preventing device destruction.

Step 1 Deposition of Back Contact for Electroplating

FIG. 2 shows the starting free-standing anodic aluminium oxide membrane10 having a front surface 12 and a back surface 14 and containing pores16 here shown as cylindrical. This first step involves depositing ametal layer on the back surface of the membrane shown schematically as aflat metal layer 36 in the structure as shown in FIG. 3. We have foundboth evaporation and sputtering to work effectively. The metal can bedeposited onto either surface but we have obtained best results forelectroplating when this metal layer is deposited onto the membranesurface which has the largest pores. Henceforth this will be termed theback surface. We have obtained good results with either copper or silverbeing used at this step.

Step 2 Electroplating

Step 2 fills the pores with a metal 18 so as to form "wires" connectingthe metal layer to the front surface of the anodic oxide film. This maybe achieved for example by standard electroplating processes which maybe used to plate out, for example Cr or W or Cu or Ag or Mo or Nb etc.in the pores. In each case a negative potential is applied to the metallayer whilst it is immersed in a suitable electrolyte. This will producethe structure shown in FIG. 4. One metal plating system we have used is:

    ______________________________________                                        327       g/l         Ni(NH.sub.2 SO.sub.3).sub.2                             14.4      g/l            NiCl.sub.2                                           30        g/l              H.sub.3 BO.sub.3                                   0.129     g/l           Sodium lauryl sulphate                                ______________________________________                                    

This solution deposits nickel in the pores and we have used currentdensities of 10 mAcm⁻² and 3 mAcm⁻² for periods of either ≈4.5 or ≈15hours respectively. We have found it advantageous to ensure that theelectrolyte only makes contact to the metal layer on the back membranesurface through the pores and not via direct contact with the metal 36.this can be achieved by holding the membrane in a water tight jig suchthat only the front surface of the membrane is directly immersed in theelectrolyte. The electrolyte then penetrates down the pores to the backsurface which is covered in metal. When the voltage is applied platingbegins in each pore. Other metals may also be plated in this way, forexample we have also obtained good results using a copper based platingsystem; however in the following it will be assumed that Ni has beenused. Plating is continued until the pores are full or nearly full. Inthis way at the end of this step the membrane may contain regions wherethe plated metal nearly reaches the membrane front surface, regionswhere it is just at that surface and regions where it has penetrated tothe surface and plating has continued to form a continuous layer ofmetal across the membrane surface. (FIG. 4).

Step 3 Mechanical Polishing

The front surface of the membrane is now polished such that a smoothfinish is produced with the nickel in the pores polished flat relativeto the surrounding alumina matrix. We normally carry out this step suchthat 10 to 20 μm of the membrane thickness is removed, by which depthover 99% of all pores are found to be full of Ni. We have found that asatisfactory finish can be achieved with the final polishing beingcarried out using 0.1 μm diamond paste. The amount of material removedin this process is not critical nor is the uniformity with which it isremoved. (FIG. 5).

Step 4 Etching Back the Metal in the Pores

The metal in the pores is now etched back from the front surface of themembrane by an amount very approximately equal to twice the diameter ofthe pores. Thus in the case of membranes with a nominal pore diameter of0.16 μm, ≈0.4 μm of metal is etched back. We have found that the etchingcan be successfully accomplished using an electropolishing technique. Inthis process the front surface of the membrane is immersed in anelectrolyte and a positive voltage is applied to the metal layerdeposited onto the back surface in step 1. We have used 4 volumes H₂ SO₄added to 3 volumes H₂ O as the electrolyte with the polishing performedat 300 mAcm⁻² current density for approximately 2 seconds or less toremove uniformly and reproducibly the required depth of Ni from 0.16 μmdiameter pores. Other etching methods may also be used for examplesputter etching or reactive ion etching or a chemical etch may be used.(FIG. 6).

It should be noted that steps 2, 3 and 4 can in principle be combined,by electrodepositing in each pore a controlled amount of metal not quitesufficient to fill the pore. in practice, it is difficult to provide auniform front end of each metal deposit in this way. So the describedtechnique, involving electroplating followed by polishing and etching iscurrently preferred.

Step 5 Deposition of Gate Layer A thin layer of metal 24 is nowdeposited at normal incidence on the front surface of the membrane. Wehave found evaporation of niobium to give particularly good results butgold, titanium and tantalum also work well. The layer thicknessdeposited is typically in the range 20-40 nm and the metal can bedeposited in stripes to facilitate matrix addressing of the emitters.Note: we have not found it necessary to deposit the gate layer at aglancing angle on a rotating substrate and thus our process is readilyscaled to large areas. (FIG. 7). The gate electrode metal also depositsat 38 on the front surface of the metal in each pore; and perhaps alsoon the pore walls as shown at 25. (The thickness of the covering overthe pore walls is greatly exaggerated in the Figures).

The next step is the first of a series of steps designed to minimise theincidence of short circuits.

Step 6 Pore Wall under-cutting

In this step the membrane is subjected to the action of a liquid whichcleans the pore walls intermediate the individual field emittingcathodes 18 and the gate electrode 24.

Preferably the membrane is now immersed in a solution of 2 g KOH in 100g of water for typically between 3 to 12 minutes and good results areobtained using a period of 8 minutes. This has the effect of removing asmall amount of alumina from the exposed pore walls so that the gatelayer slightly overhangs the surface of the pore walls at 39. Inaddition if there are small amounts of Nb left on the pore wall afterthe gate deposition step, this is removed when the alumina underneath isetched away. Thus this step serves to isolate electrically the gatelayer from the Ni deposited in the pores. (FIG. 8).

Step 7 Emitter Cone Deposition

Emitter cones are fabricated by deposition of metal perpendicular to themembrane surface. We have found that e-beam evaporation of molybdenum onto the membrane which is heated to ˜300° C. works well. Sufficient Mo isdeposited so that the pores are closed by a continuous layer 40 and thecones 20 are formed underneath. Using ˜0.16 μm diameter pores a 0.5 μmthick layer of Mo is sufficient. Some Mo may be deposited on the porewalls as shown at 31. (The thickness of the layer of Mo is greatlyexaggerated in FIGS. 9 and 10).

The apex of the cones in FIGS. 9 to 13 are shown to be coincident withthe top of the dielectric layer, it will be immediately apparent that inpractice although some of the cones terminate at this position othersmay be either higher or lower.

Step 8 Removal of Back Metal Layer

The back metal layer is now removed so that the Ni wires in adjacentpores are no longer electrically connected to each other at the backsurface. This can be done by a very light mechanical polish, e.g. 1 μmdiamond paste for ˜5 minutes, or by a chemical dissolution step or by acombination of both. If copper has been used as the back membranecontact then it can be removed by dipping in a solution of 1 part (3% H₂O₂) to 1 part ammonia. In which case the copper is rapidly removedwhereas the Ni wires appear to be untouched. In the case of silver beingused as the back metal contact a solution of 19 part sulphuric acid to 1part fuming nitric acid can be used, although in this case the Ni isattacked as well as the silver, although at a much slower rate, and sothe membrane must be left in the solution for less than about oneminutes so that very little Ni is removed. (FIG. 10).

Step 9 Removal of front metal layer and "cleaning" of pore wall

The top layer of metal on the front membrane surface which blocks thepores which contain the conical emitters is now removed. This can bedone electrolytically as we have found to work well or by means of apreviously deposited lift-off layer which is deposited between the gatelayer and the metal layer used to form the emitting cones. The lift-offlayer, if used, is dissolved in a solution which does not attack thecone material this can be done electrolytically by applying a positivevoltage to the gate layer or by chemical dissolution. We prefer not touse a lift-off layer and use either an electrolyte of 2 g KOH to 100 gof water or 4 parts sulphuric acid to 3 parts water. When dilute KOH isused as the electrolyte, a potential of 2 to 4 volts is applied to thegate layer. We have found that this cane be done effectively by applyingthe potential in bursts of 5 seconds with the potential on followed by15 seconds with the potential off. This is carried out for a totalperiod of approximately 12 minutes. This has the effect that as soon assufficient Mo is removed so that the electrolyte gains access to thepores, the potential is soon (within 5s) switched off whilst the diluteKOH solution continues to attack the exposed pore walls. In this way anysmall amount of Mo which has been deposited on these surfaces is cleanedoff so breaking electrical contact between the surface Mo layer and theMo cones. In this way when electrical isolation between the gate layerand the emitters is achieved, no further electrolytic dissolution of theMo cones can occur which are thus left with sharp points as required foroptimum field emission. The removal of Mo can also be performed by usingthe potential applied continuously for ˜3 minutes followed by ˜9 minuteswithout voltage in the KOH solution or by using the sulphuric acidsolution at ˜2 V for ˜3 minutes followed by ˜12 minutes in the KOHsolution. In each case the final step in KOH is to "clean" up thealumina walls of the pores by dissolving away some of their surface soensuring electrical isolation between the emitters and the gate layer.If a more concentrated solution of KOH is used shorter times can be usedfor this step. In some cases we found it desirable to monitor thecurrent flowing whilst the potential was applied to the gate and bynoting when this fell below some preset level it could be deduced whenall the surface Mo layer has been removed. (FIG. 11. Note that theundercutting 39 of the pores adjacent the gate electrode is morepronounced than the undercutting 39 resulting from step 6 and shown inFIGS. 8 to 10).

It is alternatively possible to apply the emitter cones 20 on the frontends of the field emitter cathodes 18 before the gate electrode 24 islaid over the front surface of the membrane. This requires the use of alift-off layer (not shown) intermediate the membrane 10 and the Mo layer40. When using this alternative, it is possible to sharpen the coneemitters by ion bombardment [2] or other means. The gate electrode 24 isthen fabricated using for example the method of Lui et al [3].

Step 10 Removal of Shorts

At this stage we have found it advantageous to introduce a further stepto mitigate against the effect of shorts between the gate layer and theemitter cones. This is because with ≈10⁸ emitters cm⁻², even after theprevious procedures we still find a small proportion of the emitters tobe shorted to the gate and with such a large density of emitters presentthis may lead to too large leakage currents in a finished device. Theeffect of any remaining shorts is removed as follows. The rear surfaceof the membrane is exposed to a solution of 4 parts sulphuric acid to 3parts water and a positive potential of ≈2 to 5 volts is applied to thefront surface of the membrane via the gate layer for ≈2 to 10 minutes.In this way any Ni wires in electrical contact with the gate layer (evenvia a resistance in excess of 1000 Mohms) are electrolytically dissolvedfrom the back surface of the membrane. Ni wires isolated from the gatelayer are not effected by this process. When a contact layer issubsequently deposited on the back of the membrane it will not makecontact with those Ni wires which were etched back electrolytically andso the effect of the shorts is removed. However the metal layer willcontact any Ni wires electrically isolated from the front surface. Byinspection of the back surface of the membrane using SEM after this stephas been completed it can be seen that ≈1% of Ni wires have been etchedback, with several μm of material removed from each, thus indicatingthat that was the proportion still shorted to the gate layer after theprevious processing. If, immediately after this step a back metalcontact is deposited it is found that the resistance between this layerand the gate layer is typically between 2 and 20 Mohms mm⁻². Howeversubsequent handling of the device and application of voltages can leadto new shorts occurring which may reduce this value to the order 5-10kohms mm⁻².

This step is shown in FIG. 12. In this example a conducting particle 32has brought the cone emitter of an individual field emitting cathodeinto electrical contact with the gate electrode. The bottom end of thatcathode has been selectively dissolved away at 34.

Step 11 Deposition of Back Surface Resistance Layer

In order to overcome the problem of accidentally induced shorts whichmay occur during use or handling of the membrane, and also to increasethe proportion of emitters that emit at any one time during operation,it is advantageous to apply a resistive layer (26, FIG. 13) to the backsurface. A layer ≈0.1 to 2 μm thick would be effective and we have tried≈1 μm thick layers of silicated diamond-like-carbon with a resistivityof 110 ohm cm and also polymer layers of MEH-PPV(p-poly(2-methoxy-5-(2-ethylhexoxy)-phenylenevinylene)) whoseresistivity was less well defined. In this way a resistance of about 10Mohms to about 300 Mohms, e.g. 30-100 Mohms, is formed between the endof each Ni wire and the final metal contact layer applied in the nextstep.

Step 12 Deposition of back surface contact layers to perform matrixaddressing

The final step is to deposit the back metal contacts (28, FIG. 1). Wehave used evaporated aluminium or evaporated silver contacts but othermetals and deposition methods such as sputtering would also work. Weapply these contacts in the form of stripes perpendicular to the stripesof the gate layer metallisation so that matrix addressing may beperformed simply.

In the above preferred embodiment of the method, steps 6, 9, 10 and 11are all designed to reduce the incidence of short circuits. Step 6 isperformed before Mo emitter cones are applied to the front ends of fieldemitting cathodes; step 9 is performed after application of the emittercones. Both steps are designed to clean the walls of the poresintermediate the field emitting cathodes and the overlying gateelectrode. Although either step may be omitted, preferably both stepsare carried out. Step 10 addresses the electrical contact between theback end of the field emitting cathode and the address electrode, and isdesigned to deal with any remaining shorts. Step 11, the deposition of aback surface resistance layer, is designed to minimise the damagingeffect of any remaining short circuits. Although one or more of thesesteps may be omitted, it is preferred that all four be included.

In tests field emission from samples produced by the method describedoccurred at 8 V or above. The emitted current varied between samples andincreased with increased voltage.

I claim:
 1. A field emitter device comprising a porous dielectric anodic metal oxide layer (10) having a front surface (12) and a back surface (14), an array of pores (16) extending through the anodic metal oxide layer from the front surface to the back surface, the pores containing wires (18) having back ends (22) and front ends constituting individual field emitting cathodes (20), a gate electrode (24) comprised of an electrically conducting material overlying the front surface of the anodic metal oxide layer wherein material of the gate electrode is substantially not present within overlying walls of the pores, and an address electrode (28) overlying the back surface of the anodic metal oxide layer and in electrical contact with the back ends of the wires, wherein there is a low or zero incidence of short circuits between the address electrode and the gate electrode, and wherein the front ends of the individual field emitting cathodes are approximately level with the front surface of the anodic metal oxide layer.
 2. The device as claimed in claim 1, wherein a resistive layer (26) is present between the back ends of the wires and the address electrode.
 3. The device as claimed in claim 1, wherein an individual field emitting cathode has a front end which is pointed and is spaced from the walls of the pore and from the gate electrode.
 4. The device as claimed in claim 3, wherein metal of an individual field emitting cathode is substantially not present overlying the walls of the pore.
 5. A field emitter device comprising a porous dielectric anodic metal oxide layer (10) having a front surface (12) and a back surface (14), an array of pores (16) having walls extending through the anodic metal oxide layer from the front surface to the back surface, the pores containing wires (18) having back ends (22) and front ends constituting individual field emitting cathodes (20), a gate electrode (24) comprised of an electrically conducting material overlying the front surface of the anodic metal oxide layer, wherein said pore walls between said gate electrode and said individual field emitting cathodes are free of said electrically conducting material, and an address electrode (28) overlying the back surface of the anodic metal oxide layer and in electrical contact with the back ends of the wires, wherein the front ends of the individual field emitting cathodes are approximately level with the front surface of the anodic metal oxide layer.
 6. A method of making a field emitter device by the steps of:a) providing a porous dielectric anodic metal oxide layer (10) having a front surface (12) and a back surface (14) and an array of pores (16) extending through the anodic metal oxide layer from the front surface to the back surface, b) providing wires (18) in the pores having back ends (22) and front ends to constitute individual field emitting cathodes (20), c) providing a gate electrode (24) comprised of an electrically conducting material overlying the front surface of the anodic metal oxide layer, and d) providing an address electrode (28) overlying the back surface of the anodic metal oxide layer and in electrical contact with the back ends of the wires,characterized by subjecting the product of step c) to a liquid which cleans pore walls intermediate to the individual field emitting cathodes and the gate electrode, to reduce the extent of short circuits between the address electrode and the gate electrode and to provide a device wherein material of the gate electrode is substantially not present within overlying walls of the pores, and wherein the front ends of the individual field emitting cathodes are approximately level with the front surface of the anodic metal oxide layer.
 7. The method as claimed in claim 6, wherein the pore wall cleaning step is performed before emitter cones (20) are formed on the front ends of the wires.
 8. The method as claimed in claim 6, wherein the pore wall cleaning step is performed after emitter cones (20) have been formed on the front ends of the wires.
 9. The method as claimed in claim 6, wherein a resistive layer (26) is provided between the back ends of the wires and the address electrode.
 10. The method as claimed in claim 6, wherein the dielectric anodic metal oxide layer is comprised of aluminum oxide, the wires are comprised of nickel having emitter cones comprised of molybdenum, and the gate electrode is comprised of niobium or titanium or tantalum.
 11. A method of making a field emitter device by the steps of:a) providing a porous dielectric anodic metal oxide layer (10) having a front surface (12) and a back surface (14) and an array of pores (16) having walls extending through the anodic metal oxide layer from the front surface to the back surface, b) providing wires (18) in the pores having back ends (22) and front ends to constitute individual field emitting cathodes (20), c) providing a gate electrode (24) comprised of an electrically conducting material overlying the front surface of the anodic metal oxide layer, d) removing electrically conducting material from the pore walls between the gate electrode and the individual field emitting cathodes, and e) providing an address electrode (28) overlying the back surface of the anodic metal oxide layer and in electrical contact with the back ends of the wires,wherein the front ends of the individual field emitting cathodes are approximately level with the front surface of the anodic metal oxide layer. 